Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of communicating the signals over long distances with very low loss. An optical network may be configured to combine modulated signals at various wavelengths or optical frequencies (also known as “channels”) into a single optical fiber. Each disparate channel may include optically encoded information to be communicated throughout the optical network. Such combining of various channels into a single fiber is known as wavelength-division multiplexing (WDM).
An optical signal may have phase noise due to finite line width of the laser providing a source of electromagnetic energy for a fiber. Such laser phase noise combined with dispersion may limit transmission performance of high spectral efficient optical QAM signal in coherent optical communication systems by causing equalization-enhanced phase noise at the coherent receiver. Phase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform, caused by time domain instabilities that are sometimes known “jitter.” Signal degradation due to laser phase noise combined with chromatic dispersion may be significant, especially when optical signals are created using a distributed feedback (DFB) laser.
Traditional approaches to solving the problem of phase noise-induced crosstalk include the use of lasers with narrow line widths. However, narrow line width lasers are often more expensive and complicated than wider line width lasers, and thus, may often be undesirable due to such cost.